1. Field of the Invention
The present invention pertains generally to a joint prosthesis and, more particularly, to a system that measures forces on a joint prosthesis to determine proper implantation of the prosthesis on a patient.
2. Description of the Prior Art
The human knee is the single largest joint of the human body, but due to its structure, is arguably the most vulnerable to damage. The leg consists principally of a lower bone called a tibia and an upper bone known as the femur. The tibia and femur are hinged together at the knee joint. The knee joint includes femoral condyles supported in an engagement with crescentic fibrocartilages that are positioned on the upper end of the tibia and receive the femur. The joint is held together by numerous ligaments, muscles and tendons. The patella is a similarly supported bone positioned in front of the knee joint and acts as a shield for it.
When the knee joint has been severely damaged from accident, wear, or disease, partial or total knee replacement may be the only viable solution. One type of knee replacement is shown in U.S. Pat. No. 4,340,978 issued to Buechel et al. In this patent, the tibia is resected to form a flat, horizontal platform known as a tibial plateau. The amount of bone structure removed corresponding to the severity of damage to the joint and the necessary allowance needed for the prosthesis. A tibial platform is secured to the tibial plateau with posts or anchors fixed normal or perpendicular to the tibia plateau. The anchors provide additional support to the tibial platform when the joint is subjected to shear, tipping and torque forces present under normal knee articulation.
A femoral component, comprising a curved convex semi-spherical shell, covers the femoral condyles and slidably engages a concave tibial bearing insert. On a side opposite the femoral component, the tibial insert is substantially flat and slidably engages the tibial platform. Interaction of opposing surfaces of these three elements, the femoral component, the tibial insert and the tibial platform allows the prosthesis to function in a manner equivalent to a natural knee joint.
Another tibial platform and a surgical procedure for implantation is described in U.S. Pat. No. 4,822,362 issued to Walker et al.
Crucial to either the complete joint of Buechel et al. or the tibial platform of Walker et al. is proper alignment of the tibial platform on the tibial plateau. Without proper alignment, neither will function correctly whereby uneven forces on the prosthesis may result in excessive contact stresses leading to deformation and/or early wear and thus undesirable short prosthetic life.
Template assemblies have been used in implantation surgical procedures to resect the tibia and align the tibial platform. One such assembly is disclosed in U.S. Pat. No. 4,211,228 issued to Cloutier. This assembly comprises a Y-shaped handle having two flat prongs that are used to check the planes of the resected tibia for overall flatness and to hold temporarily the tibia inserts. An alignment rod, fixed to the flat handle, is aligned visually along the long axis of the tibia, as viewed laterally and anteriorally, to ensure correct positioning of the tibial platform onto the patient's tibia. Since tibial platform alignment does not include movement of the prosthetic components in order to access force loads on the joint, alignment of the tibial platform may not be optimum, realizing pressure differences across the surface of the platform which under normal articulation of the joint may cause fatigue in the prosthesis.
Consequently, there exists a need for a system to dynamically measure and analyze forces present on components of a knee joint prosthesis and all other types of prostheses. The system should measure these forces throughout the normal range of motion of the joint, providing quantitative indications of forces present. The system should be easy to install and yet be removable when the analysis is complete.